Organic materials have recently shown promise as the active layer in organic based thin film transistors and organic field effect transistors [see H. E. Katz, Z. Bao and S. L. Gilat, Acc. Chem. Res., 2001, 34, 5, 359]. Such devices have potential applications in smart cards, security tags and the switching element in flat panel displays. Organic materials are envisaged to have substantial cost advantages over their silicon analogues if they can be deposited from solution, as this enables a fast, large-area fabrication route.
The performance of the device is principally based upon the charge carrier mobility of the semi-conducting material and the current on/off ratio, so the ideal semiconductor should have a low conductivity in the off state, combined with a high charge carrier mobility (>1×10−3 cm2V−1 s−1). In addition, it is important that the semi-conducting material is relatively stable to oxidation i.e. it has a high ionisation potential, as oxidation leads to reduced device performance.
Regioregular head-to-tail poly(3-hexylthiophene) has been reported with charge carrier mobility between 1×10−5 and 4.5×10−2 cm2 V−1 s−1, but with a rather low current on/off ratio between 10 and 103 [see Z. Bao et al., Appl. Pys. Lett., 1996, 69, 4108]. This low on/off current is due in part to the low ionisation potential of the polymer, which can lead to oxygen doping of the polymer under ambient conditions, and a subsequent high off current [see H. Sirringhaus et al., Adv. Solid State Phys., 1999, 39, 101].
A high regioregularity leads to improved packing and optimised microstructure, leading to improved charge carrier mobility [see H. Sirringhaus et al., Science, 1998, 280, 1741-1744; H. Sirringhaus et al., Nature, 1999, 401, 685-688; and H. Sirringhaus, et al., Synthetic Metals, 2000,111-112, 129-132]. In general, poly(3-alkylthiophenes) show improved solubility and are able to be solution processed to fabricate large area films. However, poly(3-alkylthiophenes) have relatively low ionisation potentials and are susceptible to doping in air.
It is an aim of the present invention to provide new materials for use as semiconductors or charge transport materials, which are easy to synthesize, have high charge mobility, good processibility and oxidative stability.
Another aim of the invention is to provide new semiconductor and charge transport components, and new and improved electrooptical, electronic and electroluminescent devices comprising these components, like field effect transistors (FET) as components of integrated circuitry or of thin film transistors (TFT), and organic light emitting diode (OLED) applications like electroluminescent displays or backlights of liquid crystal displays.
Other aims of the invention are immediately evident to those skilled in the art from the following description.
The inventors have found that these aims can be achieved by providing mono-, oligo- and polymers of thieno[2,3-b]thiophene as claimed in the present invention.
G. Koβmehl et al., Makromol. Chem. 1982, 183, 2747-2769 discloses poly(thieno[2,3-b]thiophene-2,5-divinylenearylenes) and methods of their preparation from corresponding aldehyde monomers, but does not disclose compounds according to the present invention.